1. Field of the Invention
The present invention relates to the energization of electroluminescent lamps and particularly to the exercise of control over the voltage applied to a capacitive load, especially a light-emitting capacitor, to reliably cause the generation of light. More specifically, this invention is directed to a driver for an electroluminescent (EL) lamp and especially to an inverter circuit which provides an alternating energization voltage having a potential and frequency commensurate with the brightness and expected service life of an electroluminescent lamp. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for application to the field of electroluminescence. Electroluminescent lamps are increasingly gaining acceptance in various products due to their unique characteristics which include thin packaging, flexibility, modest power requirements, comparatively high visibility, long life and a high level of resistance to damage. The fields of use include backlighting of LCD displays for lap top computers, directional signs, emergency and safety lighting, advertising displays, instrument lighting, etc. An electroluminescent lamp is a light-emiting capacitor which will typically comprise an electroluminescent layer sandwiched between a pair of electrodes and separated from the electrodes by dielectric layers. At least one of the electrodes will be optically transmissive. In order to stimulate the emission of light, an electroluminescent lamp must be energized by an alternating voltage having the appropriate magnitude and frequency. The service life and brightness of an electroluminescent light are a function of both excitation voltage frequency and magnitude.
The driver, i.e., the power supply, for an electroluminescent lamp will typically comprise an inverter, the lamp being connected as a capacitive load across the output of the inverter. A suitable inverter must be reliable, be capable of volumetrically efficient packaging and characterised by a circuit design which will not to impose any economic penalties which might diminish the attractiveness of employing the electroluminescent lamp for a particular application. While numerous drivers have previously been proposed for use with electroluminescent lamps, such previously available drivers have possessed one or more inherent deficiencies. A major deficiency of prior art inverters for EL applications has been that these inverters have required the use of transformers, which are relatively expensive components characterized by large space requirements. Additionally, the prior EL drivers have been characterized by the generation of an excitation voltage which is highly dependant upon load capacitance Thus, prior art electroluminescent lamp drivers have not been suitable for volumetric efficient packaging nor have they had the capability of energizing lamps of different size or characteristics with relatively constant light output. Restated, the load capacitance dependance of the prior electroluminescent lamp drivers has resulted in the output frequency and/or the output current of the drivers varying with the particular lamp to which the driver was connected.